The present invention relates to an automatic information exchange system for mobile objects, which system includes response units which are arranged at the mobile objects and each of which contains stored information regarding the associated object, and at least one interrogation unit which, when passing by a response unit, transmits a carrier signal of a first frequency to the respective response unit so as to supply operating power to the circuit elements of the response unit to enable the response unit to emit a signal modulated with the stored information at a second frequency for reception by the interrogation unit.
Such an information system equipped with interrogation and response units is disclosed in IEEE Transactions on Vehicular Technology, VOL. VT-19, No. 1, February 1970, pp. 128-136. Herein is not described a practical realization of the antennas of the response unit with which the response unit receives the carrier signal from the interrogation unit and transmits the response signal to the interrogation unit.
A response unit of an information system disclosed in German Pat. No. 2, 846,129 has a ferrite rod antenna to receive the carrier signal transmitted by an interrogation unit. However, the response unit also has a second antenna for transmitting a response signal to the interrogation unit, such signal being a signal modulated with the information stored in the response unit at another frequency than the frequency of the received carrier signal. Equipping the response units with two different antennas results in a large-area and bulky structure for the response units.